For both economic and environmental purposes it is necessary to recover silver which goes into solution in a photographic fixing process. In economic terms it is desirable to recover the maximum amount of silver from the solution which is absorbed into the solution during the fixing of photographic films and plates. The silver thus obtained is able to be re-cycled and used further in photographic processes.
It is also necessary to ensure that any discharge from photographic processing equipment into the public utility sewage and river disposal areas are such that the effluent is substantially non-polluting and meets the standards set by the utility authorities. As far as silver is concerned, it is very necessary to ensure that the silver level is kept to an absolute minimum since the presence of silver in solution can have a poisonous effect on both plant and animal life.
There are basically two known methods of recovering silver from photographic fixing solutions. The first of these is a metal-exchange system of the type such as is marketed by the Applicants, which is usually placed between the overflow from a photographic processor and a drain. One typical example of such a metal-exchange unit is disclosed in U.S. Pat. No. 3,705,716, where steel wool is used as an anode and the iron ions pass out of the steel wool into solution to replace the silver ions which are plated onto a cathode. It is necessary in this system for the electrolyte to be monitored to ensure that the silver content does not rise above a predetermined low level which would seriously affect the standard of effluent discharged into the drain. When a rise in level of silver in the effluent is detected, the process has to be stopped and the unit is disconnected and sent off for refining.
The second method which can be used is that of an electrolytic silver recovery method where no metal exchange takes place, but silver from the solution is plated onto a cathodic electrode. It is very necessary to be able to control the electrolytic method to ensure that no action takes place which causes sulphiding. Sulphiding is where a sulphide of silver is caused to deposit from the electrolyte which would destroy the fixer solution itself and reduce the quality of the plated silver.
It has been recognised previously that extremely accurate control of the electrolysis conditions are necessary if sulphiding is to be avoided. For example, suggestions have been made in U.S. Pat. No. 4,263,108 to take samples of the cell condition periodically when the cell is in a zero current state and to apply a reference voltage and to monitor this voltage so that sulphiding is minimised. This requires an interruption in the plating process periodically.
A further silver recovery process using anodic and cathodic tanks separated by a diaphragm is known from U.S. Pat. No. 4,111,766. In this patent used solution from an overflow tank is fed to the anodic and cathodic tanks and subjected, on a batch basis, to electrolytic treatment.
An alternative suggestion has been made in U.S. Pat. No. 4,377,456 to provide a control electrode of pure carbon which provides a constant reference voltage.
It has been proposed that the electrolysis takes place in the main processing tank and that removable rotating cathodes are used. The purpose of the rotating cathode is to ensure that fresh fixing solution is properly circulated to the cathodic electrodes and that there are no areas of low concentration of silver or high concentration of sulphide created in the solution and that the whole solution is subject to treatment.
If the whole of the solution is being treated in the main fixing tank the electrodes, be they rotating or fixed, have periodically to be removed from the tank and sent away to a processing unit for recovery of the silver from them. It is better if any handling of fixer-coated electrodes is avoided.
Furthermore, it is necessary to check the concentration of the silver in the solution in the main fixing tank periodically which can mean either interrupting the process to take measurements or to operate a logging system to ensure that the number of films or area of film material treated does not become too high for the fixing solution to work effectively.